Optically variable devices of various types are used as efficient anti-copying means on security- and value-documents. Among these, particularly important optically variable copy-protection means are the optically variable inks (OVI®; EP-A-0227423). Optically variable inks (OVI®) are used to print surfaces and/or indicia which exhibit a viewing angle dependent color (=color shift).
Said anti-copying inks comprise optically variable pigments (OVP™); preferred types being the flake shaped thin-film optical interference pigments described in U.S. Pat. No. 4,705,300; U.S. Pat. No. 4,705,356; U.S. Pat. No. 4,721,271 and thereto related disclosures.
Magnetic optically variable pigments in printing inks or coatings allow for the production of magnetically induced images, designs and/or patterns through the application of a corresponding magnetic field, causing an orientation of the magnetic optically variable pigment in the coating, followed by drying/curing the latter. The result is a fixed magnetically induced image, design or pattern in an optically variable ink. Depending on their layout, said images, patterns or designs are perceived as having a three dimensional or relief-like appearance, although the printing itself remains geometrically flat.
The magnetic optically variable printing inks disclosed in the co-pending European patent application EP06113891.3 of the same applicant, the respective content of which is herein included by reference, are particularly adapted for the production of magnetically induced images.
Materials and technology for the orientation of magnetic particles in coating compositions, and corresponding printing processes have been disclosed in U.S. Pat. No. 2,418,479; U.S. Pat. No. 2,570,856; U.S. Pat. No. 3,791,864; U.S. Pat. No. 3,676,273; U.S. Pat. No. 5,364,689; Us 6,103,361; US 2004/0051297; US 2004/0009309; US 2002/0160194; WO 02/09002; US 2005/0106367; WO 04/007095; WO 2005/058608 and WO 2005/002866, the respective contents of these documents are incorporated herein by reference.
The documents of the prior art provide thus various methods and apparatuses for producing a magnetic image coating on a substrate. In all cases, said magnetic image coating is provided in such a way that it can be viewed only from a single side of the substrate, noteworthy from the printed side. No recto-verso use of a magnetically oriented coating has been disclosed.